Add Bookmarks example plugin

Adds packages/plugins/examples/plugin-bookmarks-example, a new first-party
"core surface" example that exercises plugin database namespaces, local
folders, scoped API routes, and the page/sidebar/dashboard/settings UI
slots end-to-end. Registers the example as a bundled plugin in
server/src/routes/plugins.ts and excludes its workspace importer in
pnpm-workspace.yaml so PRs don't have to commit lockfile entries for it.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/plugins/examples/plugin-bookmarks-example/.gitignore

@@ -0,0 +1,3 @@
+dist
+node_modules
+.paperclip-sdk